Comet is seen after a long period of time. Why?

A comet is seen only when it comes closer to the sun. Due to sun heat comet's frozen part changes into gaseous atmosphere and seems like a bright tail. And revolution of a comet around the sun takes very long period. For some comets it is around 30 years while for some comets it is more than 30 million (30,000,000) years. That is why a comet is seen after a long period of time.